1. Technical field
The present invention relates to an analog-to-digital converter suitable for electronic control of a camera.
2. Prior Art
As cameras are automated in operation, the brightness and range information of objects to be photographed are required to be converted to digital signals. For example, as shown in Japanese Laid-Open Patent Publication No. 60-164728, there has been proposed a digital photometric device in which an object brightness detector Rt and a capacitor C are coupled in series with each other, and clock pulses are counted from the time when a photometric process is started until the voltage across the capacitor C reaches a reference voltage.
However, the proposed device is disadvantageous in that the measuring accuracy is lowered by variations in the electrostatic capacity of the capacitor C and variations in the reference voltage. Another problem is that since a comparator, used for comparing the capacitor voltage, with the reference voltage has three transistors T1, T2, T3 connected in series with a power supply Ve as shown in FIG. 6, the power supply required has to produce a high voltage which is three time the threshold voltage of each of the transistors.